Have you tried marking the variables in the bp as editable and expose on spawn?
I’m trying to create an array of structures. It works fine, but when I go to add an element it won’t let me pick one.
In the struct I am able to change these, but not in the blueprint I am making this array in. I even tried drag and dropping, and the “targetpoint 0” field just says none
Your Blueprint itself does not know anything about your Level (and the Targetpoints you try to assign)
As soon you drop it in your Level it knows to what Level it belongs and you can add whatever you want from inside the Level Editor.
as soon as you drop what in the level?
yes I did try this
how would I go about adding the target points into the BP then?
As soon as you drop your Blueprint that holds the array you talk about into the Level.
And the second question: You don´t 
the Blueprint itself does not know in what Level it will be and if the thing you want to assign will exist there. Its impossible for the Blueprint to know where its going to Pull the Targetpoints from before it actually exists as a Instance in a Level.
I know its too late but in case someone have the same question:
You can just call GetAllActorsOfClass since TargetPoints are basically a Actor Class. If you need to filter them just give tags and filter them accordingly during adding them to the array you want.